Various electronic apparatuses such as mobile phones and personal digital assistants (PDAs) have been widely used, and it has been demanded to further reduce size and weight of the electronic apparatuses and to achieve their longer lives. Accordingly, batteries, in particular, small and light-weight secondary batteries that have ability to achieve high energy density have been developed as power sources for the electronic apparatuses.
Applications of the secondary batteries are not limited to the electronic apparatuses described above, and it has been also considered to apply the secondary batteries to various other applications. Examples of such other applications may include: a battery pack attachably and detachably mounted on, for example, an electronic apparatus; an electric vehicle such as an electric automobile; an electric power storage system such as a home electric power server; and an electric power tool such as an electric drill.
There have been proposed secondary batteries that utilize various charge and discharge principles in order to obtain battery capacity. In particular, attention has been paid to a secondary battery that utilizes insertion and extraction of an electrode reactant and a secondary battery that utilizes precipitation and dissolution of an electrode reactant, which make it possible to achieve higher energy density than other batteries such as a lead-acid battery and a nickel-cadmium battery.
The secondary battery includes a cathode, an anode, and an electrolyte. The anode contains an anode active material participating in charge-discharge reaction. The configuration of the anode active material exerts a large influence on battery characteristics. Accordingly, various studies have been conducted on the configuration of the anode active material.
More specifically, in order to achieve high safety, a particle size distribution (such as a 10% cumulative diameter and a 50% cumulative diameter) of a non-graphitizable carbon material is made appropriate (for example, refer to PTL 1). In order to achieve superior charge-discharge cycle characteristics and superior heavy-load discharge characteristics, a surface of a compound containing Si and O as constituent elements is coated with carbon (for example, refer to PTL 2). In order to achieve high discharge capacity and superior cycle characteristics, for example, an oxide of silicon and a carbon material are used in combination (for example, refer to PTL 3). In order to achieve high capacity and superior cycle characteristics, an alloyed material and a carbon material are used in combination, and a ratio of the alloyed material and the carbon material and average particle diameters of the alloyed material and the carbon material are made appropriate (for example, refer to PTL 4). For a similar purpose, an alloy material and a carbon material are used in combination, and average particle diameters of the alloy material and the carbon material are made appropriate (for example, refer to PTL 5).